A toner for electrostatic image development is used in, for example, a printer, a copying machine, and a facsimile machine. The toner for electrostatic image development is used in the image formation for imaging an electrostatic image. For example, in the image formation by an electrophotographic method, an electrostatic latent image is first formed on a photosensitive drum. Then, the electrostatic latent image is developed using a toner. Subsequently, the developed image is transferred to, for example, transfer paper, and the image is fixed by heat.
Generally, a toner for electrostatic image development is produced as follows.
A binder resin and a colorant are first dry-mixed together. If necessary, into the resultant mixture are further dry-mixed, for example, a charge control agent, a release agent, and a magnetic material. Then, the resultant mixture is melt-kneaded by means of, for example, an extruder, followed by pulverization and classification. Thus, toner particles are obtained. Such a method for producing toner particles is called a melt-kneading pulverization method. An external additive is added to the toner particles obtained by this method for the purpose of imparting to the particles various performances, such as fluidity. For example, solid fine particles of, e.g., silica are added as an external additive. Consequently, the external additive is deposited on the surface of the toner particles.
In recent years, demands have been made on images formed by a copying machine or a printer wherein the images should have higher resolution. For meeting such demands, toner particles having a narrow particle size distribution are needed. However, in the above-mentioned melt-kneading pulverization method, it is difficult to control the particle diameter of the toner particles produced. Further, a great amount of coarse powder and fine powder having a particle diameter outside the desired range are by-produced. Therefore, the method poses a problem in that by-products having a particle diameter outside the desired range must be removed by classification.
As a method for solving the above-mentioned problems, a suspension polymerization method and an emulsion polymerization flocculation method are known.
In the suspension polymerization method, a polymerizable monomer, a polymerization initiator, and a colorant and others are suspended or dispersed in an aqueous medium. Then, these components are polymerized to produce toner particles.
In the emulsion polymerization flocculation method, a polymerizable monomer is added to an aqueous medium containing a polymerization initiator and an emulsifying agent, and the resultant mixture is emulsified. A colorant and, if necessary, a charge control agent and others are added to the polymer primary particle emulsion obtained by polymerizing the polymerizable monomer, thus causing the polymer primary particles to suffer flocculation. Then, the resultant flocculate particles are fused to produce toner particles.
These methods are called a polymerization method. The polymerization method is advantageous in that it is easy to control the particle diameter of the toner particles to be produced, and in that it is possible to produce a toner which has a small particle diameter and a narrow particle size distribution and which exhibits excellent image quality. Further, the polymerization method does not include a pulverization step, and therefore a binder resin having a low softening point can be used in the method. Thus, a toner having improved low-temperature fixing properties can be produced by the polymerization method.
On the other hand, the melt-kneading pulverization method is advantageous in that it is possible to continuously feed raw materials, so that a toner can be efficiently produced. Further, the kneading apparatus used in the method exhibits a high self-cleaning effect, and therefore the type of the toner to be produced can be switched merely by changing the type of the raw material fed, making it possible to continuously produce a toner.
In contrast, when a toner is produced by a polymerization method, such as a suspension polymerization method, an emulsion polymerization flocculation method, or a dissolution suspension method, the method of producing a toner in a batch-wise manner has been employed. By changing the method in a batch-wise manner to a method in a continuous manner, the production efficiency can be improved, and further products having uniform quality can be obtained if the production conditions are stable. For this reason, a technique for continuously producing a toner by the polymerization method is being developed.
For example, patent documents 1 and 2 disclose a method for continuously producing a toner by a suspension polymerization method. Also, with respect to the emulsion polymerization flocculation method having a number of production steps, studies have been made on the method of continuously performing the production steps. For example, patent document 3 discloses a method for continuously producing polymer primary fine particles of a latex. Patent documents 4 to 9 disclose an emulsion polymerization flocculation method in which a granulation step comprising a flocculation step and a fusion step is continuously performed.
A fluid inside the continuous reaction apparatus is subjected to reaction while flowing through the path of transfer. The transfer time for the fluid inside the apparatus and uniformly mixing the fluid being transferred are important. For example, in the granulation step in the emulsion (polymerization) flocculation method, for controlling the particle diameter and particle size distribution, it is necessary to strictly control the flocculation time for particles and to uniformly mix the liquid inside the apparatus.
Patent document 4 discloses a continuous producing apparatus in which the reaction mixture is agitated using a pin type blade or an anchor blade in a single reaction vessel having no partition. However, this apparatus is disadvantageous in that it is difficult to control the path of transfer of the reaction mixture in the reaction vessel. Further, the passing time of the reaction mixture is likely to be irregular. Therefore, it is difficult to strictly control the flocculate particle diameter.
Patent document 5 discloses a continuous producing apparatus in which the reaction mixture is mixed and dispersed in a single reaction vessel while transferring the reaction mixture using a screw. The transfer of the mixture using a screw is advantageously used for stably securing the path of transfer and passing time. However, the transfer using a screw has a poor mixing shearing effect. For this reason, for example, in the case of emulsion (polymerization) flocculation method, it is difficult to uniformly disperse a flocculate mixture having a certain viscosity. Further, the shearing force of the screw is small, and therefore it is difficult to precisely control the particle diameter of the flocculate particles.
Patent documents 6 and 9 disclose a continuous producing apparatus having a plurality of partitioned spaces in a single reaction vessel. Each space formed in this apparatus has an agitating blade therein. For realizing uniform mixing and reaction, this apparatus controls the path of transfer of the reaction mixture and the time during which the reaction mixture passes through the reaction vessel.
Patent documents 7 and 8 disclose a continuous producing apparatus having a plurality of reaction vessels having an agitating blade and being connected in series. In this apparatus, a plurality of reaction vessels are arranged, and hence the path of transfer of the reaction mixture can be selected freely to some extent. Therefore, by using this apparatus, a disadvantage of nonuniformity of the transfer time can be removed to some extent. However, for obtaining the intended reaction effect, a great number of reaction vessels are needed, leading to a problem in that the apparatus inevitably is complicated or increased in size. Further, when using this apparatus, it is expected that assembling of the apparatus, dealing with problems, and maintenance of the apparatus are difficult.